Process for the production of heat resistant oxymethylene polymer

ABSTRACT

A process for producing a heat-resistant polymer consisting predominantly of oxymethylene repeating units, said process comprising submitting a high-molecular-weight polymer consisting predominantly of oxymethylene repeating units to the action concurrently of A. A CAPPING AGENT SELECTED FROM THE GROUP CONSISTING OF ACID ANHYDRIDES, OXYALKYLENE KIESTERS AND LINEAR OXYALKYLENE DIETHERS, AND B. A Lewis acid.

States atet [451 Aug. 29, 1972 PROCESS FOR THE PRODUCTION OF HEATRESISTANT OXYMEYLENE POLER Inventors: Takami Ishii, Ichihara; ShotaroSugiura, Chiba; Naohisa Takikawa,

lchihara, all of Japan Assignee: Ube Industries, Ltd., Tamaguchiken,Japan Filed: May 27, 1970 Appl. No.: 40,907

Foreign Application Priority Data June 4, 1969 Netherlands ..6908497 US.Cl. ..260/67 FP Int. Cl ..C08g 1/24, C08g 1/26 Field of Search ..260/67FP, 615.5

References Cited UNITED STATES PATENTS 7/1965 Sidi ..260/67 FPSchweitzer et al. ....260/67 FP Schweitzer et al. .260/67 FP PrimaryExaminerWilliam H. Short Assistant ExaminerL. M. Phynes Att0meyShermanand Shalloway a. a capping agent selected from the group consisting ofacid anhydrides, oxyalkylene kiesters and linear oxyalkylene diethers,and

b. a Lewis acid.

3 Claims, 6 Drawing Figures PATENTED M1829 m2 SHEET 1 'UF 3 REACTIONTIME (MIN) 5 m a Z .1 Q. $250.. @2585 to Emma 5 58 62 5 4 3 2 .l. .9$2508. 253mm to him? 5 58 62 5 REACTlON TEMPERATURE PlTE'N'Ti-il'nuczsm2 SHEET 2 UF 3 .4 .8 CATALYST CONCENTRATION 5 w 3 w. h a; 5252 9.53%.no him; $43502 2 L4 1.8 MOLECULAR WEIGHT [1) PATENTEU 3,687,898

sum 3 OF 3 FREQUENCY (cm") FREQUENCY mm" PROCESS FOR THE PRODUCTION OFEEAT RESISTANT GXYWTHYLENE POLYWR This invention relates to a processfor producing a heat-resistant polymer predominantly of oxymethylenerepeating units (hereinafter referred to simply as polyoxymethylene).More specifically, the invention relates to a process for producing aheat-resistant highmolecular-weight oxymethylene polymer by submitting ahigh-molecular homopolymer or copolymer consisting of oxymethylenerepeating units to the action concurrently of a specific capping agentand a Lewis acid to thereby form a heat-resistant high-molecular-weightoxymethylene polymer whose thermal stability has been improved whileretaining the excellent mechanical properties that are possessed by thehigh-molecularweight oxymethylene polymer.

Numerous processes are known for producing polyoxymethylene. A majorityof these are processes in which the terminal grouping of thepolyoxymethylene obtained by catalytically polymerizing formaldehyde ortrioxane is stabilized. For example, Japanese Pat. applicationpublication No. 6099/1958 (U.S. Pat. Nos. 2,964,500 and 2,998,409)discloses a method of obtaining a heat-resistant polymer by theesterification of a polymer of formaldehyde with a carboxylic acidanhydride. However, the polymer produced by such methods has weak bondswithin its main chain which are susceptible to being split by heat sinceby these methods the terminal grouping of the polymer of formaldehyde ortrioxane has been merely stabilized. Hence, the thermal stability of thestabilized polymer is not necessarily adequate, with the consequencethat it is necessary to make a further addition of a stabilizer beforeit can be of practical use. Further, for adjusting the molecular weightto that desired in these methods, much trouble and inconvenience areexperienced since the molecular weight adjustment operation must beperformed during the polymerization of the monomers.

Recently, a method has been suggested wherein polyoxyethylene isstabilized by the intrasubstitution of the polyoxymethylene main chainby the oxyethylene units by reacting polyoxymethylene with dioxolan inthe presence of a Friedel-Crafts type catalyst such as boron trifluoride(British Pat. No. 1,103,730). According to this method, the thermalstability is improved as a result of the interposing of the oxyethyleneunits in the oxymethylene repeating units but, on the other hand, thereis the drawback that the crystallinity declines and the mechanicalproperties, e.g., tensile strength, falls.

We found that when polyoxymethylene was submitted to the actionconcurrently of a specific capping agent, i.e., either an acidanhydride, oxyalkylene diester or linear oxyalkylene diether, and aLewis acid heat-resistant polyoxymethylene whose thermal stability wasnotably improved while retaining intact the mechanical properties ofhigh-molecular-weight polyoxymethylene could be obtained.

It is therefore an object of the present invention to provide aheat-resistant polyoxymethylene which, while retaining intact themechanical properties of high-molecular-weight polyoxymethylene,possesses markedly improved thermal stability.

Another object of this invention is to provide a heatresistantpolyoxymethylene possessing markedly improved thermal stability withoutinterposing in the main chain of the oxymethylene repeating units aheterogeneous repeating unit.

A further object of the invention is to provide a process for producingheat-resistant polyoxymethylene whose average molecular weight can bereadily adjusted to a range desirable for a molding resin as well aswhose range of molecular weight distribution is also readily adjusted.

Other objects and advantages of the present invention will becomeapparent from the following description.

When a high-molecular weight polyoxymethylene is submitted, inaccordance with the present invention, to the action concurrently of theaforesaid capping agent, e.g., acetic anhydride, and a Lewis acid, theseverance of the main chain of oxymethylene repeating units and the endcapping by means of an acetyl group are accomplished simultaneously,with the consequence that the adjustment of the decline in averagemolecular weight and the thermal stabilization are simultaneouslyachieved. While this reaction mechanism is not quite clear, it istheorized that the capping agent specified by the present inventionforms a complex with the Lewis acid, e.g., boron trifiuoride, and thatthis complex forms coordinate bonds at the carbon and oxygen atoms ofthe main chain of oxymethylene repeating units and thereafter theseverance of oxymethylene main chain and the acetylation of one of theends and the acetoxidation of the other end are simultaneouslyaccomplished as a result of intramolecular rearrangement. It is knownthat a zipping reaction of the starting polyoxymethylene takes placewhen polyoxymethylene is submitted to the action of an acid catalyst. Itis however possible according to the present invention to adjust theaverage molecular weight of the thermally stabilized polyoxymethylene aswell as the molecular weight distribution within the ranges desiredwithout setting up substantially any zipping reaction from the severedends by submitting the polyoxymethylene to the actions of the aforesaidcapping agent and Lewis acid concurrently. Further, since thestabilization according to the present invention is carried out withoutinterposing in the main chain of oxymethylene repeating units anyheterogeneous repeating units which might cause a disturbance to thecrystalline structure, the film formed from this heat-resistant polymeris extremely tough and is no less than that formed fromhigh-molecular-weight polyoxymethylene.

The homopolymer or copolymer consisting of highmolecular-weightoxymethylene repeating units to be used in the invention process isimposed no special restriction, but those of the highest possiblemolecular weight are preferred. This is because the main chain ofoxymethylene repeating units of the starting polymer is severed at anumber of intermediate points, with the consequence that the molecularweight of the polyoxymethylene after its severance must be a valuesufficient that the shaped articles made from thepolyoxymethylene-obtained by practicing this invention demonstratephysical properties which make the article suitable for practical use.It is generally preferred that the molecular weight of the startingpolymer, as indicated by its inherent viscosity [1;], is at least 1, andparticular in the range of 3 l2.

As the starting polymer, aside from the homopolymer of eitherformaldehyde, trioxane or tetraoxane, the copolymer of eitherformaldehyde, trioxane or tetraoxane with a monomer copolymerizabletherewith, e.g., cyclic ethers, cyclic acetals or ketones, can be used.However, from the standpoint of the mechanical properties of theintended heat-resistant polymer, it is preferable that at least 90 molpercent of the polymer chain units are composed of oxymethylene units.The most suitable starting polymer is polyoxymethylene.

The capping agent that is used in the invention process is selected fromthe group consisting of acid anhydrides, oxyalkylene diesters and linearoxyalkylene diethers. As the acid anhydrides, the anhydrides ofaliphatic carboxylic acids of two to 18 carbon atoms, e.g., acetic acid,propionic acid, butyric acid, valeric acid, caproic acid, capric acid,lauric acid and stearic acid, or the anhydrides of aromatic carboxylicacids, e.g., benzoic acid, toluic acid and phenylbenzoic acid, can beused. The carboxylic acids are preferably monovalent. The oxyalkylenediesters that are used in the present invention can be indicated by thefollowing formula wherein R is alkylene, and particularly methylene orethylidene, R, and R are each alkyl or aryl group of up to 18 carbonatoms, and n is an integer from 1 to 10.

As examples of these oxyalkylene diesters, mention can be made of suchas methylene diacetate, methylene dipropionate, dioxymethylenediacetate, dioxymethylene dipropionate, trioxymethylene diacetate,trioxymethylene dipropionate, tetraoxymethylene diacetate,tetraoxymethylene dipropionate and methylene dibenzoate. On the otherhand, the linear oxyalkylene diethers that are used in the presentinvention are compounds of the formula R o Ro R wherein R is alkylene,particularly methylene or ethylidene, R, and R are each alkyl or arylgroups of up to 18 carbon atoms, and n is an integer from 1 to 10.Examples of these compounds include such as trioxymethylenedimethylether, tetraoxymethylene dimethylether, trioxymethylenediethylether, tetraoxymethylene diphenylether, dimethylethers oflowmolecular weight polyoxymethylene and dimethylethers ofoxymethylene/oxyethylene copolymer.

As Lewis acids, useable in the present invention are (a) theFriedel-Crafts type halides, e.g., BF BCl BBr Bi AlCl TiCh, SnCh, SnBrFeCl ZnCl and SbCl or (b) the complexes of said halides with ethers,esters, cyclic ethers, acid anhydrides, alcohols, ketones and water. Itis, of course, possible to use a wide range of Lewis acids aside fromthose illustrate above, but it goes without saying that these Lewisacids must be those which are capable of forming a complex with theaforesaid capping agent.

In the present invention the foregoing capping agent and Lewis acid maybe added separately to the reaction system, or it is also possible tofirst form a complex of the capping agent and Lewis acid, followingwhich the so formed complex is added to the system. At this time it isusually preferred to combine the Lewis acid with an excess of thecapping agent.

While the invention reaction can be carried out in either the vapor orliquid phase, the generally preferred practice is to carry out thereaction in the liquid phase by using either the capping agent itself oranother inert solvent as the reaction solvent. Useable as the inertsolvent are the aliphatic hydrocarbons such as n-hexane, n-heptane,n-octane, isooctane, decane and naphtha, the alicyclic hydrocarbons suchas cyclohexane and methylcyclonexane, the aromatic hydrocarbons such asbenzene, toluene, xylene and ethylbenzene, the halogenated hydrocarbonssuch as carbon tetrachloride, dichloroethane, trichloroethylene,chlorobenzene and dichlorobenzene, the esters such as ethyl acetate,butyl acetate and ethyl acetoacetate, the ketones such as methyl ethylketone, acetone and cyclohexanone, and the ethers such as ethyl ether.

The reaction conditions will vary depending upon the class of thestarting polymer, the capping agent and the Lewis acid used, as well asthe average molecular weight of the intended polymer. However, generallyspeaking, the Lewis acid is preferably present in a concentration of l/millimole per liter 100 millimoles per liter, and particularly 0.1 l0millimoles per liter, in the case of the liquid phase reaction.

The capping agent is preferably used in an amount of at least 1 percentby weight, and particularly l0 100 percent by weight, of the startingpolymer. The concentration of the capping agent in the reaction systemis preferably at least 1 percent by weight, and particularly 20 50percent by weight. The starting polymer can be reacted in a homogeneoussystem by dissolving in a reaction medium or it can also be reacted in astate of suspension in the reaction medium and, in this case, it ispreferred that the concentration of the starting polymer in the reactionmedium is in a range of l 99 percent, and particularly 5 20 percent.

There is no particular restriction as to the reaction temperature, butusually preferred is a range 50 to 180 C., and particularly 20 C. Thereaction pressure may range from normal to superatmospheric pressure,for example, 1 300 atmospheres (gauge), and particularly 1 5Oatmospheres (gauge). In carrying out the reaction it is effective toreduce the partial pressure of formaldehyde by building the pressure ofthe reaction system up to l 100 kg/cm with an inert gas, say, nitrogen.The reaction time will vary depending upon the temperature and thecatalyst concentration but a time of l 200 minutes, and particularly 560 minutes, is sufficient.

Thus is obtained in accordance with the invention process apolyoxymethylene whose molecular weight and distribution thereof havebeen adjusted to the range desired and, in addition, whose heatresistance has been greatly improved while retaining intact theexcellent mechanical properties of high-molecular-weightpolyoxymethylene. Hence, by utilizing the correlation between thereaction time, catalyst concentration and reaction temperature, and theaverage molecular weight of the resulting polyoxymethylene, it becomespossible by choosing the foregoing reaction conditions to produce, apolyoxymethylene having the desired average molecular weight andmolecular weight distribution.

For a better understanding of the present invention, reference is had tothe accompanying drawings, wherein:

FIG. 1 is a graph depicting the relationship between the reaction timeand the inherent viscosity (molecular weight) of the resulting polymer,as obtained from of the experiment of Example 1;

FIG. 2 is a graph illustrating the relationship between the reactiontemperature and the molecular weight of the resulting polymer, asobtained from the experiment of Example 4;

FIG. 3 is a graph showing the relationship between the catalystconcentration and the inherent viscosity of the resulting polymer, asobtained in Example 5;

FIG. 4 is a graph depicting the infrared spectra of the polyoxymethylenebefore practicing the invention process (FIG. 4-H) and of thepolyoxymethylene after practicing the invention process (FIG. 4-A);

and FIG. 5 is a graph indicating the molecular weight distribution ofthe polyoxymethylene obtained by the invention process.

As is apparent from FIG. 1 3, the average molecular weight of thestabilized polymer decreases when either the reaction temperature or thecatalyst concentration is raised or when the reaction time is prolonged.On the other hand, in the opposite case the average molecular weight ofthe stabilized polymer can be maintained at a relatively high level.Hence, it is possible in accordance with the invention process to adjustthe average molecular weight as well as its distribution in an optionalrange by a suitable choice of the reaction temperature and time, and thecatalyst concentration, irregardless of the average molecular weight ofthe starting polyoxymethylene, provided that the average molecularweight of this starting polymer is sufficiently high.

In the conventional methods of stabilizing the ends of thepolyoxymethylene that have been practiced in the past, the thermalstability of the resulting stabilized polymer was not yet fullysatisfactory. In addition, since the adjustment of the average molecularweight and the distribution thereof had to be carried out during thepolymerization step, its control was exceedingly difficult. However,according to invention process, the adjustment of the average molecularweight and the distribution thereof of the stabilized polymer isexceedingly simple as hereinbefore noted, provided that the averagemolecular weight of the starting polymer is sufiiciently high.

Further, as apparent from FIG. 4, it is seen that in the infraredabsorption spectrum of the polyoxymethylene subsequent to practicing theinvention an absorption at 1,740 cm" attributable to the ester groupappears anew with marked intensity, and the relative values of theabsorptions at 3,450 3,470 cm attributable to the hydroxyl groupdeclines conspicuously.

It is believed that the polymer end severed and formed anew by theformation of a coordination bond in the main chain of the startingpolyoxymethylene by the Lewis acid capping agent complex (an aceticanhydride complex of BB, in this case) has been substantially completelyesterified.

Further, it can be appreciated from FIG. 5 that, even though startingmaterials having different molecular weights are used, the molecularweight distribution of the polyoxymethylene after having been submittedto the present invention demonstrates a normal distribution in allcases, thus showing that the invention process is a very easy method foradjusting the average molecular weight and molecular weightdistribution.

The reaction in accordance with the invention process proceedssuccessively and hence the reaction can be stopped at any point desired.The reaction can be stopped readily by the addition of a stop agent tothe reaction system. As the stop agent, included are:

I. the alcohols such as methanol, ethanol, propanol, butanol, octanol,cyclohexanol, ethylene glycol, propylene glycol, diethylene glycol andbenzyl alcohol, or water;

2. the amines such as mono-, diand trimethylamines, ethylamines,propylamines, butylamines, hexylamines triethyldiamine,hexamethylenediamine, ethanolamine, ethyleneimine and polyethyleneimine;

3. the amine oxides such as tri-n-butylamine oxide and triethylarnineoxide;

4. the phosphines such as triphenylphosphine and phosphine oxide;

5. the ureas such as urea, thiourea, monoor dialkyl or aryl-substitutedurea, or the derivatives thereof;

6. the lactams such as beta-propiolactam, gammabutyrolactam,delta-valerolactam and epsiloncaprolactam;

7. the lactones such as beta-propiolactone, gammabutyrolactone,delta-valerolactone and epsiloncaprolactone; and

8. the ketones such as cyclohexanone, methyl ethyl ketone, acetone andacetyl acetone. Certain of these stop agents, e.g., ketones, can also beused as the reaction medium, provided that they do not raise thebasicity of the reaction medium excessively. The addition of the stopagent inan amount at least equivalent to the Lewis acid will do but ispreferably added such that the concentration becomes at least 1 percent.

For a further understanding of the present invention, the followingexamples are given. The values for the reaction rate constant forthermal degradation at 222 C. k inherent viscosity [1;] and toughness inthe following examples were measured in accordance with the proceduresdescribed in Japanese Pat. application publication No. 9794/1957 (US.Pat. 2,768,994, British Pat. No. 753,299).

That is to say, the reaction rate constant for thermal degradation whichis an indication of thermal stability was obtained in the followingmanner. About 1 gram of the specimen polymer was weighed, and the amountof thermal degradation of this polymer was continuously followed up asit was being heated by means of a vapor bath of methyl salicylate whilepassing nitrogen through it. The reaction rate constant for thermaldegradation was obtained from the inclination of the curve of thethermal degradation time vs, the residual weight percent of the polymerplotted from the amount of thermal degradation that was measured for 30minutes at l-minute intervals.

Since this degradation reaction can be regarded as being for the mostpart a first order reaction, the reaction rate constant for thermaldegradation k was expressed in percent per minute.

and phenylamines,

"Dioxolan-inserted polyoxyrnethylene prepared in accordance with themethod of British Pat. No. 1,103,730 (Example 3) using as the startingpolyoxymethylene the same polyoxymethylene used in Example 6.

We claim:

1. A process for producing a film-forming heat-resistant polymer, whichcomprises contacting a starting polyoxymethylene homopolymer offormaldehyde, trioxane or tetraoxane concurrently with a. a cappingagent in an amount of at least lpercent by weight based on the weight ofthe starting polymer, and selected from the group consisting of i. ananhydride of an aliphatic mono-carboxylic acid of two to 18 carbonatoms, and ii. an oxyalkylene diester of the formula 0 0 R1( io-R0-limwherein R is a methylene group, R, and R each being selected from thegroup consisting of alkyl and aryl groups of up to 18 carbon atoms; and(b) a Lewis acid, in a concentration of from 1/100 m mol to m mol perliter of reaction medium, and selected from the group consisting of i. ahalide selected from the group consisting of 81 BCl BBr Bi AlCl TiCL,SnCl SnBr FeCl AnCl and SbCl, and

ii. a complex of said halide with ethers, esters, cyclic ethers, acidanhydrides, alcohols, ketones and water, at a temperature ranging from-50 to C. for a period of from 1 to 200 minutes in the liquid phase.

2. The process of claim 1 wherein said Lewis acid is selected from thegroup consisting of boron trifluoride and complexes of borontrifluoride.

3. The process of claim 1 which comprises adding to the reaction system,after submitting said polyox-

2. The process of claim 1 wherein said Lewis acid is selected from thegroup consisting of boron trifluoride and complexes of borontrifluoride.
 3. The process of claim 1 which comprises adding to thereaction system, after submitting said polyoxymethylene polymer to theaction of the capping agent and said Lewis acid, a stop agent selectedfrom the group consisting of (a) alcohol or water, (b) amines, (c) amineoxides, (d) phosphines, (e) urea, thiourea, mono- or dialkyl oraryl-substituted ureas, (f) lactams (g) lactones and (h) ketones to stopthe reaction.